Such sensing devices, mounted on a chip at the end of a catheter, are commonly used for measuring blood pressure. One example is described in WO 9727802. Such a prior art chip 1 mounted at the end of a catheter/guide wire (from here on referred to as a catheter) 18 is schematically shown in FIG. 1a. A pressure sensing element 3 is in this example a piezoresistor 3 mounted on a pressure sensitive diaphragm 5. There could also be more than one piezoresistor mounted on the diaphragm for measuring the pressure. Since piezoresistors are very sensible for temperature variations it is common to place a temperature sensing element 7 close to the pressure sensing element 3. This temperature sensing element 7 is often also a piezoresistor 7 of the same type as the pressure sensing element 3, but it is placed essentially unaffected by pressure, i.e. not on a pressure sensing element. The measuring values obtained from the temperature sensing element 7 are used for compensating the pressure measurements for the temperature effect.
A wiring diagram common for these measurements is shown in FIG. 1b. The diagram comprises both the components on the chip 1 in FIG. 1a and means R, Us, Uout placed somewhere at the other end of the catheter 18, located outside the body during measurements. As apparent in FIG. 1b, it is common to connect the temperature sensing element, Rt, 7 and the pressure sensing element, Rp, 3 to one branch each in a Wheatstone bridge 8 where two further resistors 9,11 are located each in one bridge branch at the other end of the catheter 18, i.e. outside the body during measurements. This implies that three conductors 12a,12b,12c have to be connected to the chip 1 (shown in FIG. 1a). The conductors 12a,12b,12c are soldered onto one terminal pad 13a,13b,13c each. The pads are in turn connected to the piezoresistors 3, 7. The conductors 12a,12b,12c are isolated from each other and enclosed in the catheter 18 leading from the chip 1 to outside the body.
Another way of reducing the temperature offset in a pressure sensor is described in U.S. Pat. No. 5,715,827. The method used here is to provide two pressure sensitive piezoresistors on the chip and connected to a circuit outside the chip so as to form a Wheatstone bridge with one piezoresistor in each one of two of the bridge branches. This coupling corresponds to the one shown in FIG. 1b and thus three conductors are needed out from the chip. The two piezoresistors are both sensitive for pressure but they change their resistances with opposite signs due to pressure changes. Because of the coupling in a Wheatstone bridge the resistance changes due to pressure changes are additive and the resistance changes due to temperature changes are subtractive. This implies that if the piezoresistors are perfectly matched the temperature dependence will cancel out.
The soldering of the wires to the pads is an expensive process. Actually the mounting cost of the chip is many times bigger than the material cost in the chip. Also it is a problem that the three wires have to be isolated from each other in the catheter and that the catheter needs to be very thin.